Plasma display devices using PDPs (Plasma Display Panels) have the advantage that thinning and larger screens are possible. In the plasma display devices, images are displayed by utilizing light emission in the case of gas discharges.
FIG. 17 is a diagram for explaining a method of driving discharge cells in an AC PDP. As shown in FIG. 17, the surfaces of electrodes 301 and 302 opposite to each other are respectively covered with dielectric layers 303 and 304 in the discharge cell in the AC PDP.
As shown in FIG. 17(a), when a voltage lower than a discharge start voltage is applied between the electrodes 301 and 302, no discharges are induced. As shown in FIG. 17(b), when a voltage in a pulse shape (a write pulse) higher than the discharge start voltage is applied between the electrodes 301 and 302, discharges are induced. When the discharges are induced, negative charges are stored in a wall surface of the dielectric layer 303 after moving in the direction of the electrode 301, and positive charges are stored in a wall surface of the dielectric layer 304 after moving in the direction of the electrode 302. The charges stored in the wall surface of the dielectric layer 303 or 304 are called “wall charges”. Further, a voltage induced by the wall charges is called a “wall voltage”.
As shown in FIG. 17(c), the negative wall charges are stored in the wall surface of the dielectric layer 301, and the positive wall charges are stored in the wall surface of the dielectric layer 302. In this case, the polarity of the wall voltage is opposite to the polarity of an externally applied voltage. Accordingly, an effective voltage in a discharge space drops as the discharges progress, so that the discharges are automatically stopped.
As shown in FIG. 17(d), when the polarity of the externally applied voltage is inverted, the polarity of the wall voltage is the same as the polarity of the externally applied voltage. Accordingly, the effective voltage in the discharge space rises. When the effective voltage at this time exceeds the discharge start voltage, discharges which are opposite in polarity to the discharges shown in FIG. 17(b) are induced. Consequently, the positive charges move toward the electrode 301, to neutralize the negative wall charges which have already been stored in the dielectric layer 303. The negative charges move toward the electrode 302, to neutralize the positive wall charges which have already been stored in the dielectric layer 304.
As shown in FIG. 17(e), the positive and negative wall charges are respectively stored in the wall surfaces of the dielectric layers 303 and 304. In this case, the polarity of the wall voltage is opposite to the polarity of the externally applied voltage. Accordingly, the effective voltage in the discharge space drops as the discharges progress, so that the discharges are stopped.
Furthermore, as shown in FIG. 17(f), when the polarity of the externally applied voltage is inverted, discharges which are opposite in polarity to the discharges shown in FIG. 17(d) are induced. Consequently, the negative charges move toward the electrode 301, and the positive charges move toward the electrode 302. The program is then returned to the state shown in FIG. 17(c).
After the discharges are thus started once by applying the write pulse higher than the discharge start voltage, the discharges can be continued by inverting the polarity of the externally applied voltage (a sustain pulse) lower than the discharge start voltage using the function of the wall charges. To start discharges by applying a write pulse is called address discharges, and to continue discharges by applying sustain pulses which are alternately inverted from each other is called sustain discharges.
As shown in FIG. 17(g), it is possible to cause the wall charges stored in the wall surface of the dielectric layer 303 or 304 by applying an erasure pulse which is opposite in polarity to the wall voltage between the electrodes 301 and 302 to disappear, to terminate discharges. The pulse width of the erasure pulse is set to a small width such that remaining wall charges can be canceled and the wall charges which are opposite in polarity to the remaining wall charges cannot be newly stored. When the wall charges disappear once, no discharges are induced even if the subsequent sustain pulse is applied, as shown in FIG. 17(h).
FIG. 18 is a schematic view mainly showing the configuration of a PDP (Plasma Display Panel) in a conventional plasma display device.
As shown in FIG. 18, a PDP 1 comprises a plurality of address electrodes 11, a plurality of scan electrodes (scanning electrodes) 12, and a plurality of sustain electrodes (maintenance electrodes) 13. The plurality of address electrodes 11 are arranged in the vertical direction on a screen, and the plurality of scan electrodes 12 and the plurality of sustain electrodes 13 are arranged in the horizontal direction on the screen. The plurality of sustain electrodes 13 are connected to one another.
A discharge cell is formed at each of the intersections of the address electrodes 11, the scan electrodes 12 and the sustain electrodes 13. The discharge cell constitutes a pixel on the screen.
An address driver 2 drives the plurality of address electrodes 11 in response to image data. A scan driver 3 successively drives the plurality of scan electrodes 12. A sustain driver 4 together drives the plurality of sustain electrodes 13.
FIG. 19 is a schematic sectional view of a three-electrode surface discharge cell in the AC PDP.
In a discharge cell 100 shown in FIG. 19, a scan electrode 12 and a sustain electrode 13 which are paired with each other are formed in the horizontal direction on a front glass substrate 101. The scan electrode 12 and the sustain electrode 13 are covered with a transparent dielectric layer 102 and a protective layer 103. On the other hand, an address electrode 11 is formed in the vertical direction on a back glass substrate 104 opposite to the front glass substrate 101. A transparent dielectric layer 105 is formed on the address electrode 11. A fluorescent member 106 is applied on the transparent dielectric layer 105.
In the discharge cell 100, a write pulse is applied between the address electrode 11 and the scan electrode 12 so that address discharges are induced between the address electrode 11 and the scan electrode 12. Thereafter, periodical sustain pulses which are alternately inverted from each other are applied between the scan electrode 12 and the sustain electrode 13 so that sustain discharges are induced between the scan electrode 12 and the sustain electrode 13.
An ADS (Address and Display period Separated) system is used as gray scale expression in the AC PDP. FIG. 20 is a diagram for explaining the ADS system. The vertical axis in FIG. 20 indicates the scanning direction of the scan electrodes (the vertical scanning direction) corresponding to the first line to the m-th line, and the horizontal axis indicates the time.
In the ADS system, one field ( 1/60 seconds=16.67 ms) is divided into a plurality of sub-fields on a time basis. For example, when 256 gray scale expression is made by eight bits, one field is divided into eight sub-fields. Each of the sub-fields is separated into an address period during which address discharges for selecting cells which are to be turned on are induced and a sustain period during which sustain discharges for display are induced.
In the example shown in FIG. 20, one field is divided into four sub-fields SF1, SF2, SF3, and SF4 on a time basis. The sub-field SF1 is separated into an address period AD1 and a sustain period SUS1, the sub-field SF2 is separated into an address period AD2 and a sustain period SUS2, the sub-field SF3 is separated into an address period AD3 and a sustain period SUS3, and the sub-field SF4 is separated into an address period AD4 and a sustain period SUS4.
In the ADS system, scanning by address discharges is performed on the whole surface of the PDP from the first line to the m-th line in each of the sub-fields. When the address discharges on the whole surface are terminated, sustain discharges are induced. That is, the sustain period is set in a period excluding the address period. Therefore, the ratio of the sustain period occupied in one field is decreased to approximately 30%, so that there is a limit to luminance improvement.
In order to increase the luminance of the PDP, therefore, an address-while-display scheme (TECHNICAL REPORT OF IEICE.EID96-71, ED96-149, SDM96-175 (1997-01),PP.19-24) is proposed. FIG. 21 is a diagram for explaining the address-while-display scheme. The vertical axis in FIG. 21 indicates the scanning direction of the scan electrodes (the vertical scanning direction) corresponding to the first line to the m-th line, and the horizontal axis indicates the time.
In the address-while-display scheme, sustain discharges are started subsequently to address discharges for each of the lines. In the example shown in FIG. 21, one field is divided into four sub-fields SF1, SF2, SF3, and SF4. The sub-fields SF1 to SF4 respectively include address periods AD1 to AD4 and sustain periods SUS1 to SUS4.
The sustain periods SUS1 to SUS4 are set subsequently to the address periods AD1 to AD4 for each line. Therefore, almost all of one field is a sustain period, which allows luminance improvement.
FIG. 22 is a timing chart showing a voltage for driving each electrode by a conventional address-while-display scheme. In FIG. 22, voltages for driving a sustain electrode 13, scan electrodes 12 corresponding to the n-th line to the (n+3)-th line, and an address electrode 11, where n is an arbitrary integer.
In FIG. 22, sustain pulses Psu are applied to the sustain electrode 13 in a predetermined period. In an address period, a write pulse Pw is applied to the scan electrode 12. Write pulses Pwa are applied to the address electrode 11 in synchronization with the write pulse Pw. The on-off of the write pulses Pwa applied to the address electrode 11 is controlled depending on each of pixels composing a displayed image. When the write pulse Pw and the write pulses Pwa are simultaneously applied, address discharges are induced in a discharge cell at the intersection of the scan electrode 12 and the address electrode 11, so that the discharge cell is turned on.
In a sustain period after the address period, sustain pulses (maintenance pulses) Pse are applied to the scan electrode 12 in a predetermined period. The phase of the sustain pulses Psc applied to the scan electrode 12 is shifted 180° from the phase of the sustain pulses Psu applied to the sustain electrode 13. In this case, sustain discharges are induced only in the discharge cells which have been turned on by the address discharges.
When each of the sub-fields is terminated, an erase pulse Pe is applied to the scan electrode 12. Consequently, wall charges in each of the discharge cells disappear, so that the sustain discharges are terminated. In a time period elapsed from the time when the erase pulse Pe is applied until the subsequent sub-field is started, suspended pulses Pr are applied to the scan electrode 12 in a predetermined period. A period elapsed from the time when the erase pulse Pe is applied until the subsequent sub-field is started is referred to as a suspended period.
In the above-mentioned conventional address-while-display scheme, the sustain pulses Psu are always applied to the sustain electrode 13 in a predetermined period, and the sustain pulses Psc or the suspended pulses Pr are always applied to the scan electrode 12 in a predetermined period. Accordingly, power consumption is increased by charge or discharge currents in the sustain electrode 13 and the scan electrode 12.
An object of the present invention is to provide a display device in which power consumption is reduced and a method of driving the same.